Observations and analysis of electrokinetically driven particle trapping in planar microelectrode arrays

In situ observations of submicron fluorescent tracers suspended in high ionic strength media sealed in a confined geometry are combined with 3‐D simulations in order to provide a better understanding of the synergism between dielectrophoresis and electrothermal flows that cause rapid particle transport and trapping on the surface of planar quadrupolar microelectrodes. The influence of electrode design on the microfluidic patterns and observed particle collection is examined by employing two different types of microelectrodes in the experiments. The potential use of quadrupolar microelectrodes as means for achieving accelerated sampling and signal amplification in future surface based biosensor devices is illustrated with an experiment involving stable capture of antigen‐coated polystyrene particles on the surface of an antibody‐functionalized microelectrode array.     

Rare Cell Capture in Microfluidic Devices

This article reviews existing methods for the isolation, fractionation, or capture of rare cells in microfluidic devices. Rare cell capture devices face the challenge of maintaining the efficiency standard of traditional bulk separation methods such as flow cytometers and immunomagnetic separators while requiring very high purity of the target cell population, which is typically already at very low starting concentrations. Two major classifications of rare cell capture approaches are covered: (1) non-electrokinetic methods (e.g., immobilization via antibody or aptamer chemistry, size-based sorting, and sheath flow and streamline sorting) are discussed for applications using blood cells, cancer cells, and other mammalian cells, and (2) electrokinetic (primarily dielectrophoretic) methods using both electrode-based and insulative geometries are presented with a view towards pathogen detection, blood fractionation, and cancer cell isolation. The included methods were evaluated based on performance criteria including cell type modeled and used, number of steps/stages, cell viability, and enrichment, efficiency, and/or purity. Major areas for improvement are increasing viability and capture efficiency/purity of directly processed biological samples, as a majority of current studies only process spiked cell lines or pre-diluted/lysed samples. Despite these current challenges, multiple advances have been made in the development of devices for rare cell capture and the subsequent elucidation of new biological phenomena; this article serves to highlight this progress as well as the electrokinetic and non-electrokinetic methods that can potentially be combined to improve performance in future studies.     

Hydrodynamics of Geldart group A particles in gas-solid fluidized beds

Geldart group A particles were fluidized in a 10 cm i.d.×1.8 m high Plexiglas-made bed with ambient air to determine the hydrodynamic properties in a gas-solid fluidized bed. The effects of static bed heights, position of pressure measuring points, differential and absolute pressure fluctuations on the hydrodynamic behavior of a Geldart group A particles in a gas-solid fluidized bed were investigated. The particles used in this study were 80 micrometer FCC powders and 60 micrometer glass beads. The variance of pressure fluctuations was used to find the minimum bubbling velocity. The obtained minimum bubbling velocity was compared with the other methods available in the literature. This method was found to be much easier and had better data reproducibility than the classical visual method or sedimentation method. The variance of pressure fluctuations increased due to the increase of superficial gas velocity and static bed height. The obtained minimum bubbling velocity and pressure fluctuations were found to depend on the measuring position along the axial direction. The effect of measuring position was discussed. Cross-correlation of two pressure signals was used to find the delay time, then the bubble rising velocity.     

A microfluidic toolbox for cell fusion

Cellular fusion is a key process in many fields ranging from historical gene mapping studies and monoclonal antibody production, through to cell reprogramming. Traditional methodologies for cell fusion rely on the random pairing of different cell types and generally result in low and variable fusion efficiencies. These approaches become particularly limiting where substantial numbers of bespoke one‐to‐one fusions are required, for example, for in‐depth studies of nuclear reprogramming mechanisms. In recent years, microfluidic technologies have proven valuable in creating platforms where the manipulation of single cells is highly efficient, rapid and controllable. These technologies also allow the integration of different experimental steps and characterisation processes into a single platform. Although the application of microfluidic methodologies to cell fusion studies is promising, current technologies that rely on static trapping are limited both in terms of the overall number of fused cells produced and their experimental accessibility. Here we review some of the most exciting breakthroughs in core microfluidic technologies that will allow the creation of integrated platforms for controlled cell fusion at high throughput. © 2015 Society of Chemical Industry     

Particle aggregation and capture by walls in a particulate aerosol channel flow

A soft-sphere discrete-element method is used to examine particle aggregate formation and particle capture by walls in a laminar channel flow. Adhesive particulates have been identified as a leading cause of failure in many different microfluidic devices, including those currently being developed by different research groups for rapid biological and chemical contaminant sensing, fluid drag reduction, etc. As these microfluidic devices enter into the marketplace and become more extensively used in field conditions, the importance of particle adhesion and clogging will increasingly limit the reliability of such systems. At a larger scale, clogging of vehicle radiators by small adhesive particles is currently a major problem for construction vehicles operating in various environmental conditions and soil types. Cooling system fouling leads to the need for frequent maintenance and machine down time. Dust fouling of equipment is also of concern for potential human occupation on dusty planets, such as Mars. The paper provides a detailed investigation of the fundamental mechanics leading to adhesion of particle aggregates to channel walls, which involves a combination of aggregate capture, aggregate deformation by particle rolling, and shearing of aggregates from the wall. Cases with different adhesion potential, particle sizes, and flow Reynolds number are examined, with both single-size particles and a binary particle mixture.     

Separation of Cells and Cell-Sized Particles by Continuous SPLITT Fractionation Using Hydrodynamic Lift Forces

Abstract

The use of hydrodynamic lift forces for the separation of particles according to size by continuous SPLITT fractionation is explored. The mechanism for particle separation in the transport mode of SPLITT fractionation is first explained. This is followed by a discussion of the hydrodynamic lift forces that act upon particles entrained in fluid flow between the parallel bounding walls of the SPLITT cell. The effect of the bounding walls on particle motion both parallel and perpendicular to the direction of flow is explained. Computer simulations of particle trajectories are presented that predict extremely high size selectivity for the method. A parallel experimental study was carried out using both polystyrene latex particles and red blood cells. The experimental selectivity was found to be smaller than that predicted theoretically. This discrepancy is attributable to nonidealities in the construction of the SPLITT cell. Nonetheless, the results are promising. Suspensions of polystyrene particle standards (from 2 to 50 μm in diameter) demonstrate that fast and relatively clean size separations are possible provided particles differ sufficiently in size and flow conditions are properly optimized. It is also shown that the system has the potential to quickly and gently separate blood cells from plasma.     

Ion–Aerosol Flux Coefficients and the Steady-State Charge Distribution of Aerosols in a Bipolar Ion Environment

Fuchs’ theory, as corrected by Hoppel and Frick, is widely used to compute flux coefficients of ions to aerosol particles and the resultant charge distribution. We have identified approximations made in previous works that limit the theory's accuracy. Hoppel and Frick used two characteristic speeds or kinetic energies to calculate the flux coefficients of ions to aerosol particles in lieu of an average of the flux coefficients over the Maxwell–Boltzmann distribution of ion speeds. In the present work, we show that this approximation artificially reduces the number of multiply charged particles. Ion capture may be enhanced by three-body trapping, a process wherein an ion has a collision with a neutral gas molecule and loses sufficient kinetic energy to be captured by the particle. The gas kinetic theory approach to three-body trapping has been refined to better account for the collision between the ion and a neutral gas molecule within the potential presented by the particle. Approximations to the calculation of energy losses and the probability of ion capture have been relaxed. The possibility that an image charge may be induced on the ion as well as on the particle is allowed. While the previous work was limited to electrically conductive particles, both the ion and the particle are allowed to have any dielectric constant in the present work, and the finite size of the ions is taken into account when calculating minimum capture radii for the ion–particle interactions. The resulting ion flux coefficients differ from previous results both in the low nanometer regime and in the continuum regime. We explore the influence of key parameters on the charge distribution, including dielectric constant, temperature, and pressure, to understand how operating conditions may affect the interpretation of differential mobility analyzer measurements of particle size distributions. Finally, an empirical expression for the new charge distribution is given to facilitate rapid calculations.

© 2013 American Association for Aerosol Research     

Rapid Concentration of Nanoparticles with DC Dielectrophoresis in Focused Electric Fields

We report a microfluidic device for rapid and efficient concentration of micro/nanoparticles with direct current dielectrophoresis (DC DEP). The concentrator is composed of a series of microchannels constructed with PDMS-insulating microstructures for efficiently focusing the electric field in the flow direction to provide high field strength and gradient. The location of the trapped and concentrated particles depends on the strength of the electric field applied. Both ‘streaming DEP’ and ‘trapping DEP’ simultaneously take place within the concentrator at different regions. The former occurs upstream and is responsible for continuous transport of the particles, whereas the latter occurs downstream and rapidly traps the particles delivered from upstream. The performance of the device is demonstrated by successfully concentrating fluorescent nanoparticles. The described microfluidic concentrator can be implemented in applications where rapid concentration of targets is needed such as concentrating cells for sample preparation and concentrating molecular biomarkers for detection.     

Extending the EMMS/bubbling model to fluidization of binary particle mixture: Formulation and steady-state validation

The EMMS/bubbling model originally proposed for fluidization of monodisperse particles is extended to fluidization of binary particle mixture in this study. The dense and dilute phases are considered to comprise of two types of particles differing in size and/or density. Governing equations and the stability condition are then formulated and solved by using an optimization numerical scheme. The effects of bubble diameter are first investigated and a suitable bubble diameter correlation is chosen. Preliminary validation for steady state behavior shows the extended model can fairly capture the overall hydrodynamic behaviors in terms of volume fraction of bubbles and average bed voidage for both monodisperse and binary particle systems. This encourages us to integrate this model with CFD for more validations in the future.     

Microfluidic fabrication of smart microgels from macromolecular precursors

Stimuli-responsive polymer microgels can be produced with exquisite control using droplet microfluidics; however, in existing methods, the droplet templating is strongly coupled to the material synthesis, because droplet solidification usually occurs through rapid polymerization immediately after the microfluidic droplet formation. This circumstance limits independent control of the material properties and the morphology of the resultant microgel particles. To overcome this limitation, we produce sensitive polymer microgels from pre-fabricated precursor polymers. We use microfluidic devices to emulsify semidilute solutions of crosslinkable poly(N-isopropylacrylamide) and solidify the drops via polymer-analogous gelation. This approach separates the polymer synthesis from the particle gelation and allows each to be controlled independently, thus enabling us to form monodisperse, thermo-responsive microgel particles with well-controlled composition and functionality. In addition, the microfluidic templating allows us to form complex particle morphologies such as hollow gel shells, anisotropic microgels, or multi-layered microgel capsules.     

Capture of Viral Particles in Soft X-Ray–Enhanced Corona Systems: Charge Distribution and Transport Characteristics

The charge distribution of airborne MS2 bacteriophage nanoparticles and the efficiency of electrical-mobility–based capture mechanisms with bipolar charging were studied. MS2 virions form large agglomerated particles in a suspension. The average charge on airborne MS2 virions can be as high as one unit charge (negatively charged). The application of both soft X-ray irradiation and alpha rays from a Po-210 bipolar charger was shown to not only reduce the average charge on MS2 virion particles but also partially fragment the larger MS2 virion agglomerates, thereby increasing the number of ultrafine MS2 virion particles. A cylindrical electrostatic precipitator with a mounted soft X-ray emitter was used to determine the effectiveness of electrical capture methods for virus particles. At low applied voltages, it was found that the capture efficiency of ultrafine virus particles can be increased by applying in situ soft X-ray irradiation with electrostatic precipitation. It has also been shown that in the presence of both a positive and negative corona, virus particles are readily captured with log removal values exceeding 4. The unit developed and demonstrated in this work is a compact, low-pressure drop system that can be readily mounted in ventilation ducts or air supply systems to remove ultrafine particles such as viruses.     

Electrical and thermal characterization of a dielectrophoretic chip with 3D electrodes for cells manipulation

This paper presents a characterization of a DEP device with 3D electrodes fabricated using microtechnology. A characteristic of the proposed DEP device is that the microchannel walls are made from heavy-doped silicon, acting simultaneously as electrodes. Theoretical analysis shows that the structure with 3D electrodes presents a uniform DEP force in the cross-section of the microfluidic channel, hence producing a higher trapping efficiency compared to those classical DEP devices with thin planar electrodes. Numerical simulation using finite element method (ANSYS) has demonstrated that with 3D silicon electrodes, the change in temperature is 8-10 times lower as compared to those classical DEP devices with thin planar electrodes. Experimental results show that DEP device with 3D silicon electrodes has a better trapping efficiency of cells, hence providing a great potential for high volume cells manipulation.     

Estimate of confidence intervals for geometric mean diameter and geometric standard deviation of lognormal size distribution

Confidence of particle size distribution, which is the size distribution of sample particles selected from a large population with lognormal size distribution, has been studied theoretically. Theoretical equations were derived from the basic formulas commonly used in statistics to estimate confidence intervals for geometric mean diameter and geometric standard deviation. Computer simulation has provided size distribution of sample particles by random sampling in order to confirm the theoretical equations. For both geometric mean diameter and geometric standard deviation, the confidence intervals were calculated so that both values of population were placed approximately in the middle of the intervals. The tendencies for the intervals to decrease with an increase in sample particle number and/or significance level, and with a decrease in geometric standard deviation, were reasonable in statistics. The proposed theoretical equations should be useful for estimating confidence of lognormal size distribution.     

Phase hold-up and critical fluidization velocity in a three-phase inverse fluidized bed

We studied the hydrodynamic characteristics of a three-phase inverse fluidized bed made of a transparent acrylic column of 0.115 m inner diameter and 2 m heights. Air, water and polyethylene particles were used as the gas, liquid and solid phase, respectively. We used both hydrophobic low density polyethylene (LDPE) and hydrophilic LDPE as solid phase, and distilled water as liquid phase, and filtered air as gas phase. The LDPE was chemically treated by chlorosulfonic acid to change the surface property from hydrophobic to hydrophilic. We tried to solely investigate the effect of the surface hydrophilicity of polymeric particles on the phase holdup and the critical fluidization velocity of three-phase inverse fluidization. Thus, we measured the static pressure and eventually observed critical fluidization velocity. Critical fluidization velocity became smaller in case of using MDPE hydrophobic particles than LDPE hydrophilic particles. This was thought to be due to the retardation of rising bubbles near hydrophobic particles and, subsequently, the increase of gas hold-up.     

Hydrodynamic Cell Model: General Formulation and Comparative Analysis of Different Approaches

This paper is concerned with the Cell Model method of addressing hydrodynamic flow through system of solid particles. The starting point of the analysis is the general problem formulation intended for describing a pressure driven flow through a diaphragm which can be considered as a set of representative cells having arbitrary shape and containing any number of particles. Using the general problem formulation, the hydrodynamic field inside an individual representative cell is interrelated with the applied pressure difference and the external flow velocity. To this end, four relationships containing integrals over the outer boundary of a representative cell are derived in the paper. Assuming that the representative cell is a sphere containing a single particle in the centre, the derived general relationships are transformed into outer cell boundary conditions employed in the literature by different authors. The general number of the obtained outer boundary conditions is more than the required number. Accordingly, by choosing different sets of the outer boundary conditions, different models are considered and compared with each other and with the results obtained by others for regular particle arrays. The common and different features of the hydrodynamic and electrodynamic versions of the Cell Model approaches are analyzed. Finally, it is discussed which version of the cell model gives the best approximation while describing pressure and electrically driven flows through a diaphragm and sedimentation of particles.     

Particle-Substrate Collisions, Particle Rebound and Removal

Recent progress in particle capture and rebound and its effect on the adhesion force is reviewed in this paper. Particles rebound when the incident velocity is greater than a characteristic critical velocity. Lower impaction velocity particles experience elastic and plastic deformation. Recent models for particle rebound and capture are discussed and evaluated in terms of their restrictive assumptions and results. Recent experimental data of particle rebound and capture is also discussed, as is the hydrodynamic removal of captured particles. The removal of particles occurs when the applied hydrodynamic removal force overcomes the adhesion force. The effect of adhesion-induced deformation on the removal of particles is introduced and discussed.     

介电电泳分离稀土氧化物微粒(英文)

A challenge in chemical engineering is the separation and purification of rare-earth elements and their compounds. We report the design and manufacture of a dielectrophoresis (DEP) microchip of microelectrode arrays. This microchip device is constructed in order to use DEP to capture micro-particles of rare-earth oxides in petroleum. Dielectrophoretic behavior of micro-particles of rare-earth oxides in oil media is explored. The dielectrophoretic effects of particles under different conditions are investigated. It is showed that the prepared microchip is suitable for use in the investigation of dielectrophoretic responses of the rare-earth oxides in oil media. The factors such as frequency, particle size and valence of rare-earth metal are discussed. When the frequency is fixed, the translation voltage decreases as particle size increases. Lower frequencies are more effective for manipulation of inorganic particles in oil media. Particles of the same rare-earth oxide with different size, as well as particles of different rare-earth oxides, are captured in different regions of the field by regulating DEP conditions. This may be a new method for separation and purification of particles of different rare-earth oxides, as well as classification of particles with different size.     

Production of PDMS microparticles by emulsification of two phases and their potential biological application

Poly(dimethylsiloxane) (PDMS) has been widely used for prototyping of chips. Among many kinds of microstructures, it is almost impossible to make few micron-sized PDMS particles through microfluidic focusing methods because of the highly viscous property of uncured PDMS. Vigorous mixing of PDMS with water resulted in small, spherical particles was found. Furthermore, a simple and easy method was devised to assure an emulsion state. The PDMS–water emulsion was made using two syringes connected by a needle. This emulsion was rapidly heated to cure the PDMS within the emulsion. The resultant PDMS was examined using spectroscopic methods to determine the particulate morphology and size distribution. PDMS particles of which the size was around 1?µm with a narrow size distribution was produced. DNA delivery into cultured animal cells through these PDMS microparticles was also demonstrated as an illustrative application. These biologically inert microparticles will find many more practical applications with additional fine-tuning modifications based on discussed considerations.     

Development and characterization of a “store and create” microfluidic device to determine the heterogeneous freezing properties of ice nucleating particles

Abstract

Understanding heterogeneous ice nucleation induced by ice nucleating particles (INPs) is hindered by analytical challenges in accurately determining the freezing temperature spectrum, abundance, and physicochemical properties of INPs. Here we evaluate the performance of a microfluidic device that employs a “store and create” approach to measure the ice nucleation properties of approximately 600 uniformly sized nanoliter water droplets. These droplets are immersed in surfactant-free environmentally sustainable squalene oil and do not contact the polymer walls of the microfluidic device. The device interfaced with a cold plate temperature controller has a greatly reduced background freezing temperature spectrum for filtered water droplets compared to conventional microliter droplet-on-substrate freezing methods. Droplets containing particles of interest are readily generated on-chip from a suspension of particles in water. Background freezing for 6 nL water droplets exhibits a median freezing temperature of ?33.7?±?0.4?°C, close to the theoretical freezing temperature of ?34.5?°C. The immersion freezing temperature spectra obtained from Snomax bacterial and illite mineral particles compares well with literature data, and the freezing contribution from either type of particle can be separated from a mixed suspension. Our approach generates a highly uniform droplet size distribution, causes no clogging of the microfluidic device, and is capable of reproducible droplet refreezes. The high-resolution freezing spectra obtained from large droplet number arrays enables the use of the derivative INP temperature spectrum analysis to quantitatively distinguish between different classes of INPs. The lower and consistent filtered water background freezing temperature enables measurements of almost the entire immersion freezing temperature regime from ?33 to 0?°C, and quantification of weaker but often abundant INPs such as those found in biomass-burning smoke aerosol.

Copyright © 2019 American Association for Aerosol Research